Significant improvements in the functionality of integrated circuits may be obtained by integrating different materials with specialized properties onto a base semiconductor such as crystalline silicon. For example, semiconductor structures incorporating thin films of gallium nitride, germanium, germanium-silicon, etc. on a silicon base would enable the development of transistors with integrated optics capabilities or the capability of operation at much higher frequencies than presently possible. A major obstacle to growing thin films of one semiconductor material on another (such as germanium on silicon) is the lattice mismatch and consequent strain-induced film morphology. This obstacle controls and limits the film morphology and, in turn, the electrical and optical properties of the film.
Strain induced thin film morphology limitations are encountered, for example, in the heteroepitaxial growth of silicon-germanium on crystalline silicon substrates. It is found that heteroepitaxial growth of Si1-xGex with x>0.2 typically results in the formation of three-dimensional islands which can act as quantum dots (QDs) because they localize charge. These coherently strained QDs form naturally as a strain reduction mechanism for the film. If x is less than 0.2, a strained film is formed which does not have the 3D islands. Heterojunction bipolar transistors (HBTs) have been made using such defect-free epitaxial films of silicon-germanium and have shown dramatically improved performance relative to silicon HBTs. However, it would be desirable to be able to increase the germanium concentration in such films beyond that which has been possible in the prior art because of the occurrence of the quantum dot islands at higher germanium concentrations. In addition, the thickness of a heteroepitaxial silicon-germanium film of a given germanium concentration is limited by the critical thickness at which dislocations form because of the 4.2% lattice mismatch. While it would be desirable to be able to produce thicker defect-free films for many applications such as HBTs, for certain applications, the quantum dots that are formed in strained films are desirable because of potential applications in quantum computation and communication, light detectors, and lasers. It would be preferable for many of these applications that the arrangement of quantum dots be regular, rather than random, and with a narrow quantum dot size distribution.